Metal tantalum, niobium are valve metals. An important application of tantalum particle and niobium particle is for manufacturing electrolytic capacitors. As the affinity of tantalum with oxygen is large, a thermal oxide film is always formed on the surface of metal tantalum. Capacitor grade tantalum particle has a large specific surface area and a high oxygen content, especially, the tantalum particle having a specific capacitance higher than 30,000 μF·V/g has a relative high oxygen content. The oxygen content of tantalum sintered pellet for manufacturing electrolytic capacitor is rigorously limited. Owing to that the surface of tantalum particle has a thermal oxide film of 3˜8 nanometers thick, the larger of the specific surface area of the tantalum particle, the higher the content of oxygen. In the preparation of the porous tantalum sintered pellet for the capacitor anode, when the oxygen content of tantalum sintered pellet is higher than the limit of solid solubility of oxygen in tantalum (about 2 at %, at T<873 k), a precipitated oxide phase will form on the surface of the sintered pellet. The precipitated oxide phase will precipitate on the amorphous Ta2O5 film formed when anodizing, which will lead to an electro-crystallization and the formation of nucleus and conducted area. Due to the difference of the density of the amorphous Ta2O5 and the crystalline Ta2O5, the crystallization will lead to the cracking of the amorphous Ta2O5, resulting in the degradation of the performance of the dielectric film and the lowering of the life of the capacitor manufactured with the particles.
If the surface of the tantalum or niobium particle is nitrogen-doped before the formation of the thermal oxide film, the content of oxygen of this particle could be reduced. Also, the doping of the nitrogen has a passivation action, and can reduce the fire risk. Moreover, in the preparation of the capacitor, the nitrogen-doped tantalum particle has a sintering retardant action, so the sintered shrinkage could be reduced, and the porosity could be increased, which is in favor of the impregnation of cathode material, and is in favor of the increasing of the capacitance and the decreasing of the equal series resistance (ESR) of the capacitor. In addition, the nitrogen atom can inhibit the oxygen atom migrating from tantalum oxide film to the tantalum matrix, thereby the tantalum anode containing nitrogen has low electrical leakage and has high rated voltage.
Doping nitrogen for the tantalum powder and doping nitrogen for tantalum anode in the sintering process are an important technique of improving the performance of tantalum powder and tantalum capacitor for the tantalum powder producer and capacitor manufacturer.
Chinese patent application CN 1108036A discloses a process for preparing a capacitor grade powder, electrodes, and finished capacitors there from having reduced electrical leakage characteristics. The powder is prepared by reacting Group V-B base materials with nitrogen of between 500 and 7000 ppm and oxygen of between 700 and 3000 ppm. The process was carried out in a sealed retort in which the mixture of tantalum powder and magnesium was heated up to 900° C. for 3 hours, then cooled to 400° C.˜800° C., nitrogen source selected from at least one of nitrogen gas, ammonia or magnesium nitride gas was introduced into the retort.
Chinese patent application CN 1426334A discloses a process of preparing a nitrided valve metals comprising nitriding a valve metal powder at sufficient temperature of 200˜350° C. during a heat treatment, that is prior to a deoxidation step.
Due to that the nitriding temperature of the processes above is high, tantalum and niobium react quickly with nitrogen. In addition, since the nitrogen concentration within the reactor is not uniform, thus it is hard to ensure the distribution of nitrogen between different particles to be uniform.
Chinese patent application CN 1433485A provides a process, in which while a metallic compound is reduced with a reducing agent, a nitrogen-containing gas is introduced into a reaction system to thereby form metal, and nitrogen is simultaneously incorporated into the metal. Chinese patent application CN 1872461A discloses a process of manufacturing the metal powder containing nitrogen, in which a metal salt is reduced with a reducing agent and undergoes reduction within a diluent salt, thereby generating the metal, wherein a nitrogen containing gas is introduced into the space containing the reaction melt comprising the metal compound, the reducing agent and the diluent salt, thereby generating the metal and incorporating the nitrogen within the metal. The two nitriding processes are carried out at a temperature of 800° C. or higher. In the process, when the metal compound containing the metal is reduced, the fresh metal atom generated has a strong reactivity with the nitrogen containing gas at the temperature of 800° C. or higher to form metal nitride. In addition, the nitrogen contact only with a part of the metal particles, instead of contacting the whole surface of the metal particles coated with melt salt, so the nitrogen contents between particles are not uniform. Though the following thermal treatment can make some nitrogen uniformly contained within the particle itself and among abut particles, there is a disadvantage on the uniformity of contents of nitrogen among all the particles.
WO 02/04152 A1 disclosed a process comprising a nitrogen treatment step in which a tantalum or niobium powder is heated up to 850° C. with stirring in a rotary kiln in a nitrogenous atmosphere to incorporate nitrogen into the powder. This process is also carried out at high temperature, and thus tantalum nitride is likely to form. Further, this process is complex with a high manufacturing cost. Moreover, the tantalum particles with different nitrogen concentrations have different properties. For instance, the particle having high concentration of nitrogen has high hardness, and the pellet pressed therefrom has low crush strength. It is hard to sinter the tantalum particles of high nitrogen concentration with the tantalum particles of low nitrogen concentration. The pellet pressed with the tantalum particles having different nitrogen concentration has low crush strength. Moreover, the pellet sintered therefrom has low crush strength and the sintered necks thereof are thin or big, resulting in that the capacitor manufactured from this sintered pellet has high electric leakage, poor electric performance.
In order to avoid the above-mentioned problems, it is desired that the nitrogen contents among the valve metal particles are uniform on the whole, and the particles are essentially not containing any nitride; in addition, for preparing the valve metal particles, it is needed a process in which the temperature is low so avoiding fore-mentioned problems due to a high temperature process.